1. Field of the Invention
This invention relates to a device and method for providing pumping support to an artificial ventricle or heart. More particularly, the present invention pertains to a hydraulic pumping system used in connection with an artificial ventricle which relies on transfer of hydraulic fluid between a pumping chamber associated with the ventricle and a volume displacement chamber which is separated from the pumping chamber and associated ventricle.
2. Prior Art
Current trends in research in the general field of ventricle assist devices and artificial hearts is placing greater emphasis on electrohydraulic drive systems instead of air-driven mechanisms which require use of a driveline through the skin. The advantages of an electrohydraulic heart are numerous. The total unit is implantable and self-contained. In contrast, pneumatic drive systems are cumbersome and impose serious constraints in view of the transcutaneous driveline and potential for infection.
An early design for an electrohydraulic drive system is set forth in U.S. Pat. No. 4,173,796 by Jarvik. It discloses the use of an axial impeller assembly with an electric drive motor, referred to hereafter as a "fluid pump and drive motor". Hydraulic fluid is moved by action of the impeller assembly as it rotates about its axis. By reversing the electric motor, the hydraulic fluid can be reversibly pumped, thereby filling and extracting the fluid with respect to a pumping chamber associated with the ventricle.
When used within a total artificial heart, the fluid pump and drive motor can simple be reversed to transfer fluid from a first pumping chamber associated with a left ventricle, to a second pumping chamber associated with the right ventricle. The difference in cardiac output between the left and right ventricles can be balanced by use of an intra-atrial shunt, a leaking pulmonary artery valve or a small extra compliance reservoir.
In situations where the pumping fluid is not alternately driving a blood pumping chamber, a volume displacement chamber is used to store this pumping fluid. For example, during diastole in a single ventricle assist device, pumping fluid is removed from the pumping chamber associated with the ventricle through a conduit, and is stored in a bag or other volume displacement chamber. This transfer is accomplished with the same type of reversible fluid pump and drive motor as is disclosed in the Jarvik patent. During systole, the motor reverses and forces the fluid from the displacement chamber to the pumping chamber with the ventricle.
Prior practice in positioning the fluid pump and drive motor has followed the pattern set by the Jarvik patent. Specifically, this device is placed in the intermediate flow line or interconnect between the pumping chamber and the volume displacement chamber. This is a logical position because the fluid must pass through the impeller assembly of the pump, which naturally becomes part of the flow path. Accordingly, prior art practice has consistently positioned the fluid pump and drive motor as a continuous part of the interconnect device, or as a continuous part of the interconnect flow path. As such, hydraulic fluid contact has been limited to the interior flow channel within the fluid pump. The exterior surface of the fluid pump and drive motor have been treated much like a tubular enclosure in that this exterior structure functioned to contain the fluid within the flow path. Therefore, any contact of hydraulic fluid with the exterior of the interconnect and associated fluid pump was contrary to reasonable design considerations.
The use of an electrohydraulic drive system involves other mechanical considerations which are not associated with a pneumatic drive system. For example, because the system is self-contained within the patient, dependability and durability are critical. Both of these factors are affected by the minimization of wear on components of the drive motor. Accordingly, a variety of techniques have been applied to design drive motors with a minimal amount of friction, as well as other mechanical factors that cause abrasive wear and generate attendant heat.
Although advanced technology has provided much improved motor design, there remains the challenge of dissipating heat generated with the drive motor. It will be apparent to those skilled in the art that electrohydraulic drive systems depend on conversion of electric power to hydraulic power. Such conversions will always generate some heat as a by product. When such heat is confined and accumulated within the small volume of a pumping system in support of a ventricle, some detrimental effect is inevitable. In prior art systems where the drive motor and impellers are further confined within a tubular interconnect, dissipation of heat is even more difficult. Without effective heat control, increased wear occurs and the inevitable failure of the system is accelerated.